You should look in the Oxygen Preferences->"XML / XML Parser" page if the XInclude processing checkbox is enabled.If it is and you still have problems with this, maybe you could give us more details, ideally post a small sample XML + XSD and the error message that you get on your side. You can also send samples directly to "support@oxygenxml.com".

and not supported, so if you have an element in that namespace, it is not considered an xinclude element, it is just an unknown element called "include" having the namespace https://www.w3.org/2003/XInclude.